Generally, this invention relates to rechargeable batteries. More specifically, this invention relates to prismatic rechargeable batteries.
In rechargeable electrochemical cells, weight and portability are important considerations. It is also advantageous for rechargeable cells to have long operating lives without the necessity of periodic maintenance. Rechargeable electrochemical cells are used in numerous consumer devices such as calculators, portable radios, and cellular phones. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable electrochemical cells can also be configured as larger xe2x80x9ccell packsxe2x80x9d or xe2x80x9cbattery packsxe2x80x9d.
Rechargeable electrochemical cells may be classified as xe2x80x9cnonaqueousxe2x80x9d cells or xe2x80x9caqueousxe2x80x9d cells. Examples of a nonaqueous electrochemical cell are lithium and lithium-ion cells which typically use a liquid organic or polymer electrolyte. Aqueous electrochemical cells may be classified as either xe2x80x9cacidicxe2x80x9d or xe2x80x9calkalinexe2x80x9d. An example of an acidic electrochemical cell is a lead-acid cell which uses lead dioxide as the active material of the positive electrode and metallic lead, in a high-surface area porous structure, as the negative active material. Examples of alkaline electrochemical cells are nickel cadmium cells (Nixe2x80x94Cd) and nickel-metal hydride cells (Nixe2x80x94MH). Nixe2x80x94MH cells use negative electrodes having a hydrogen absorbing alloy as the active material. The hydrogen absorbing alloy is capable of the reversible electrochemical storage of hydrogen. Nixe2x80x94MH cells typically use a positive electrode having nickel hydroxide as the active material. The negative and positive electrodes are spaced apart in an alkaline electrolyte. Preferably, an alkaline electrolyte is an aqueous solution of an alkali metal hydroxide such as such as potassium hydroxide, sodium hydroxide and lithium hydroxide.
Upon application of an electrical potential across a Nixe2x80x94MH cell, the hydrogen absorbing alloy active material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, forming a metal hydride. This is shown in equation (1): 
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released from the metal hydride to form a water molecule and release an electron.
Hydrogen absorbing alloys called xe2x80x9cOvonicxe2x80x9d alloys result from tailoring the local chemical order and local structural order by the incorporation of selected modifier elements into a host matrix. Disordered hydrogen absorbing alloys have a substantially increased density of catalytically active sites and storage sites compared to single or multi-phase crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
Some extremely efficient electrochemical hydrogen storage alloys were formulated, based on the disordered materials described above. These are the Tixe2x80x94Vxe2x80x94Zrxe2x80x94Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 (xe2x80x9cthe ""400 Patentxe2x80x9d) the disclosure of which is incorporated herein by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the ""400 Patent utilize a generic Tixe2x80x94Vxe2x80x94Ni composition, where at least Ti, V, and Ni are present and may be modified with Cr, Zr, and Al. The materials of the ""400 Patent are multiphase materials, which may contain, but are not limited to, one or more phases with C14 and C15 type crystal structures.
Other Tixe2x80x94Vxe2x80x94Zrxe2x80x94Ni alloys, also used for rechargeable hydrogen storage negative electrodes, are described in U.S. Pat. No. 4,728,586 (xe2x80x9cthe ""586 Patentxe2x80x9d), the contents of which is incorporated herein by reference. The ""586 Patent describes a specific sub-class of Tixe2x80x94Vxe2x80x94Nixe2x80x94Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The ""586 Patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them. Other hydrogen absorbing alloy materials are discussed in U.S. Pat. Nos. 5,096,667, 5,135,589, 5,277,999, 5,238,756, 5,407,761, and 5,536,591, the contents of which are incorporated herein by reference.
In order to achieve high power Nixe2x80x94MH batteries (for example, for use in hybrid electric vehicle applications), it is advantageous to increase the surface area of the electrodes as much as possible. With cylindrical cells, very high surface area can be achieved without a high parts count. Typically, one positive electrode and one negative electrode are wound together with interleaved separator layers that prevent contact of the positive and negative electrodes. The surface area can be increased by using longer and thinner electrodes without increasing the number of electrodes, which remains at two.
However, there are certain disadvantages to using cylindrically wound cells. Special techniques are needed to provide low resistance connection between terminals and the electrodes. Also, the packaging efficiency of cylindrical cells is inferior to that of prismatic cells. As well, there are also some upper limitations on the size of the cylindrical cells that can be manufactured as well as the length of electrodes that can be conveniently wound into a coil (due, at least in part, to a variation in the radius of the winding). Heat transfer can also be an issue with large cylindrical cells.
There have been attempts to build wound prismatic electrochemical cells of various types. Instead by winding around a central spindle rod, the electrodes are wound around a flat plate or fixture (and, hence, are a xe2x80x9cflat rolledxe2x80x9d configuration). The cross-section of the wound electrodes is be more of a flattened oval rather than round and would fit into a prismatic case.
In a conventional prismatic design of fixed dimensions, the electrode surface area can be increased by increasing the electrode count by using thinner electrodes. A difficulty with this approach is that the cost increases with the increased parts count that is due to the increased electrode count. Hence, there is a need for a prismatic electrochemical cell that can have a large electrode surface area with a small parts count.
One aspect of the present invention is an electrochemical cell, comprising: at least one positive electrode; at least one negative electrode; and an electrolyte, wherein the at least one positive electrode and/or the at least one negative electrode folded in a zigzag configuration.
Another aspect of the present invention in an electrochemical cell, comprising: an electrode stack including a positive electrode and a negative electrode, the electrode stack folded in a zigzag configuration having folds and creases; and an electrolyte.
Another aspect of the present invention is an electrochemical cell, comprising: an electrode folded in a zigzag configuration having folds and creases; at least one counter-electrode disposed within one or more of the folds of the electrode; and an electrolyte.
Another aspect of the present invention is an electrochemical cell, comprising: an electrode folded in a zigzag configuration having folds and creases; and at least one bifold counter-electrode having a first leg and a second leg, the first leg and the second leg disposed within a first and a second fold on the same side of the zigzag configuration of the electrode.
Another aspect of the present invention is an electrocemical cell, comprising: a bifold negative electrode having a first and a second leg; a bifold positive electrode having a first and a second leg; and an electrolyte, wherein the first leg of the negative electrode is disposed between the first and second leg of the positive electrode, and the first leg of the positive electrode is disposed between the first and second leg of the negative electrode.
Another aspect of the present invention is an electrochemical cell, comprising: an electrode stack including an electrode disposed between a first and a second counter-electrode, the electrode stack folded in a zigzag configuration with folds and creases; and an electrolyte.
Preferably, the electrochemical cells disclosed above are prismatic electrochemical cells.